Illumination for inspecting surfaces of articles

ABSTRACT

An illumination system including at least one reflector subtending an angle with respect to a location on a surface of an article, and first and second light sources, the first and second light sources each providing a light output, the light outputs from both of the first and second light sources being directed to impinge on the location on the surface of an article within the angle, at least one of the light outputs being reflected by the reflector.

RELATED APPLICATION

[0001] The present application is a division of U.S. patent applicationSer. No. 09/565,500, filed May 5, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to visual inspection of surfaces ofarticles, and more particularly to an illuminator for the automatedoptical inspection of ball grid array substrates, lead frames andprinted circuit boards.

BACKGROUND OF THE INVENTION

[0003] Apparatus and methods useful for illuminating substantially flatpatterned surfaces of articles, such as electrical circuits on printedcircuit boards (PCBs), ball grid array substrates (BGAs) reticles,semiconductors and other similar articles, during the automated opticalinspection thereof are well known in the art.

[0004] During automatic optical inspection of flat patterned surfaces ofarticles, such as electrical circuits on PCBs and BGAs, the surface isilluminated by intense broad spectrum illumination while the article istransported beneath a sensor, such as a CCD or TDI camera.Conventionally, the sensors acquire scanned gray level images of thesurface. Various materials which appear on the surface of an articleeach have different reflective properties and reflect illumination at adifferent level of intensity. For example, copper which definesconductors, various metal platings on the conductors, and the substrateitself each have different reflective properties. The reflectedintensities in the image are sensed and automatically processed andanalyzed to determine the presence of defects in the patterns on thesurfaces.

[0005] The surfaces of electrical circuits being inspected, althoughsubstantially flat, generally exhibit a topographical relief thatresults both from the cross-sectional configuration of conductors aswell as the surface microstructure thereof. Typically, very intenseillumination impinging on a surface of an article being inspected over asolid angle of incidence is employed to mitigate negative affects of thetopographical relief

[0006] The following patents are believed to represent the state of theart in high intensity illumination for the inspection of substantiallyflat patterned article surfaces such as electrical circuits on printedcircuit boards (PCBs), ball grid array substrates (BGAs), lead frames,reticles and semiconductors:

[0007] U.S. Pat. No. 4,421,410 to Karasaki describes an illuminatorcomprising a half reflecting mirror disposed above a printed wiringboard. Concentrated light is reflected off the mirror and directed ontothe wiring board at an angle substantially normal to the surface of theboard and diffuse light emanating from fiber optics is simultaneouslydirected onto the surface at a large angle of incidence. An image of aline on the surface is transmitted through the mirror to a sensor.

[0008] U.S. Pat. No. 4,877,326 to Chadwick describes a high intensityilluminator providing focused quasi lambertian illumination to a regionof the surface of an article to be inspected. The illuminator includes ahalf reflecting mirror, first and second and third ellipticalcylindrical reflectors and first, second and third lamps, all havingmutually parallel elongate axes. Two of the reflectors and two of thelight sources are spaced from each other and illuminate the surface withfocused light at a large angle of incidence. The half reflecting mirror,the third reflector, and the third lamp are arranged to reflect focusedlight along an axis normal to the surface to fill the gap between thefirst and second reflectors. Each lamp is located at one focus of areflector, and the illuminated region is located at the second focus ofthe reflectors. Forth and fifth planar reflectors are provided at thelongitudinal ends of the first, second and third reflectors, and asensor is provided to image the illuminated region by sensing lightreflected therefrom which passes through the half reflecting mirror.

[0009] Israel patent 81450 in the name of Orbotech Ltd. describes a highintensity illuminator similar in structure to that described in U.S.Pat. No. 4,877,326, but employing light supplied via fiber optics, andeffectively having a numerical aperture substantially smaller than thenumerical aperture of the apparatus described in U.S. Pat. No.4,877,326.

[0010] U.S. Pat. No. 5,058,982 to Katzir describes a high intensityilluminator comprising a beam splitter cube and first, second and thirdelliptical cylindrical reflectors, two of which are spaced from eachother. Light received via fiber optics is provided at one focus of eachreflector. The reflectors are oriented so that the second focus of eachreflector illuminates a region of a surface to be inspected. A thirdilluminator and a condensing lens are provided and oriented to directlight through the beam splitter and onto the surface to be inspectedalong an axis normal thereto. A sensor is oriented to receive lightreflected from the surface via the beam splitter cube.

[0011] U.S. Pat. No. 5,153,668 to Katzir describes an illuminator forilluminating an area to be inspected on the surface of an article, inwhich the illumination is configured to be substantially circularlysymmetric over a solid angle around an optical axis normal to thesurface. A sensor is provided to image the surface through a gap betweenthe illuminators.

[0012] U.S. Pat. No. 4,801,810 to Koso describes an ellipticalreflector, comprising approximately one half of an elliptical cylinder,which is used to illuminate the surface of a printed circuit board. Theaxis of the elliptical cylinder is oblique to the surface of the printedcircuit board. A lamp is disposed under the reflector at one focus ofthe ellipse, while the region illuminated is located at the other focus.An imaging system images the illuminated region through an apertureformed in the reflector.

[0013] The illuminator employed in Inspire™ automated optical systemsmanufactured and sold by Orbotech Ltd., and described in copending PCTapplication PCT/IL98/00285 (unpublished), is a high intensityilluminator comprising a first light source that emits light over acontinuous wide angle of illumination toward a surface of an article tobe inspected and has a blocking element that blocks a portion of thecontinuous angle of illumination to form two separate portions ofillumination. A second light source is employed to supply illuminationto the region blocked by the blocking element. Concentrating optics areprovided to concentrate the illumination onto the article. Theillumination is provided at a first angle to the normal, and an imagingsensor is provided to image the illuminated region of the article at asecond angle to the normal.

[0014] Systems which image a surface to be inspected along an imagingaxis that is normal to the surface being inspected typically employ atleast three separate sources of illumination. In systems that employless than three sources of illumination to provide an intense solidangle of illumination, the surface is imaged along an axis which isoriented at a non-normal angle thereto.

[0015] Additionally, conventional systems that illuminate and image asurface to be inspected along an axis that is normal to the surfacebeing illuminated acquire images via beam splitting apparatus thatintroduces undesired aberrations into the image.

SUMMARY OF THE INVENTION

[0016] The present invention generally seeks to provide high intensityillumination for the automated optical inspection of patterned articlesin which two light sources together provide a solid angle ofillumination, the spatial uniformity of which is adjustable.

[0017] One aspect of a preferred embodiment of the invention providesillumination of a region on the surface of an article over a solid angleusing two independently adjustable sources of illumination, wherein theaxis of at least one source of illumination is substantially normal tothe surface. Preferably, the region illuminated is imaged by a sensorwhose axis of imaging is also substantially normal to the surface.

[0018] According to another aspect of a preferred embodiment of theinvention, a region of an article to be inspected is illuminated with afirst illuminator, including a reflective surface which is apertured ata location which overlies the region to be inspected. Supplementalillumination of the region is provided through the aperture and a sensorimages the region to be inspected by sensing light reflected therefromthrough the aperture.

[0019] According to still another aspect of a preferred embodiment ofthe invention, an imaging system is provided in which the surface of thearticle is illuminated by a solid angle of illumination to illuminate aline, and wherein a sensor is provided to image the illuminated regionin a manner that avoids transmitting reflected light from the regionthrough a beam splitter or through a partially reflected mirror en routeto the sensor.

[0020] Preferably, a partially reflective planar mirror is orientedalong an imaging axis normal to the surface. Illumination is transmittedthrough the partially reflective planar mirror to provide illuminationalong an axis normal to the surface. Light reflected from the surface isreflected by the partially reflective mirror into the sensor for imagingthe surface..

[0021] In accordance with still another aspect of a preferred embodimentof the invention, an illuminator is provided to illuminate a linearregion along a surface of an article during automated optical inspectionthereof. The illuminator is configured to provide a broad angle ofillumination from a multiplicity of illumination sources. At least oneillumination source provides illumination along an axis which issubstantially normal to the surface. Illumination from that source ispassed through an optical element that introduces astigmatism into theillumination along an axis of astigmatism that is substantially colinearto the linear region, thereby smearing the illumination along theilluminated linear region.

[0022] Another aspect of a preferred embodiment of the inventionprovides an illuminator for illuminating a linear region along a surfaceof a substrate with a broad angle of illumination. The illumination isprovided by a plurality of sources, wherein a first source ofillumination provides illumination along an axis that is substantiallynormal to the surface of an article. The illumination provided by thefirst source is passed through an optical assembly configured tointroduce astigmatism into the illumination provided by the firstsource. The astigmatism is oriented along an axis that is substantiallycolinear to the linear region being illuminated. Preferably, an imagingsensor is provided to image the linear illuminated region at leastpartially along an axis that is normal to the surface of the article.

[0023] According to still another aspect of the invention, there isprovided a substantially telecentric imaging system for imaging a regionof a surface of an object during automated optical inspection thereof.The system comprises a plurality of lenses and sensors operativesimultaneously to acquire a plurality of images of mutually overlappingregions. Preferably, the maximum angle at which any pixel is imaged isless than approximately 5° and preferably less than 3.7° to the normalwith respect to the surface. The imaging path of at least one of thesensors is preferably folded by a folding mirror which is located in aregion adjacent to a waist in the imaging paths of the other sensors.

[0024] According to another aspect of the invention, there is providedan inspection system for automatically inspecting patterned articles.Illumination in a first spectral range is provided on one surface of thearticle and illumination in a second spectral range, distinguishablefrom the first range, is provided on the opposite surface of thearticle. Sensors are employed to sense the intensity of reflected lightin the first spectral range, and to sense the intensity of transmittedlight in the second spectral range. Patterns on the surface of thearticle are inspected based on reflected light intensity in the firstspectral range. The presence and pattern of desired and undesiredapertures in the article are inspected based on transmitted light in thesecond spectral range.

[0025] There is thus provided in accordance with a preferred embodimentof the present invention an illumination system including at least onereflector subtending an angle with respect to a location on a surface ofan article, and first and second light sources, the first and secondlight sources each providing a light output, the light outputs from bothof the first and second light sources being directed to impinge on thelocation on the surface of an article within the angle, at least one ofthe light outputs being reflected by the reflector.

[0026] Further in accordance with a preferred embodiment of the presentinvention the reflector is formed with a light transmissive region topermit a light output from a first one of a the first and second lightsources to pass therethrough. Preferably the light transmissive regionis an aperture formed in the reflector. Preferably the illuminationsystem also includes an optical element disposed intermediate the firstone of the first and second light sources and the aperture for directingthe light output of the first one of the first and second light sourcesthrough the aperture. Additionally the optical element also includes atleast one reflecting surface.

[0027] Alternatively the illumination system may also include an opticalassembly disposed along an optical path extending between the opticalelement and the location on the surface of an article for directly lightreflected from the location on the surface of an article to a lightsensor. Preferably the optical assembly is generally transmissive to thelight output of the first one of the first and second light sourcesdirected through the aperture onto the location on the surface of anarticle and generally reflective of light reflected from the location onthe surface of an article.

[0028] Furthermore the illumination system may also include a reflectivesurface disposed intermediate a second one of the first and second lightsources and the reflector for directing the light output of the secondone of the first and second light sources onto the reflector.

[0029] Additionally in accordance with a preferred embodiment of thepresent invention the at least one reflector is an elliptical reflectorand the second one of the first and second light sources and thelocation on the surface of an article are each located at or near alocus of the elliptical reflector. Preferably the at least one reflectorincludes two spaced apart reflectors which together define a section ofan ellipse. The spaced apart reflectors are preferably separated by anaperture.

[0030] Alternatively the first and second light sources may also beindependently controllable light sources.

[0031] There is also provided in accordance with a preferred embodimentof the present invention an illumination system including a cylindricalreflector extending along a first longitudinal axis and having an axialaperture formed therein which extends along the longitudinal axis, afirst elongate light source extending along a second longitudinal axis,generally parallel to the first longitudinal axis and being arranged toilluminate the cylindrical reflector, such that light from the firstelongate light source is directed onto a plane generally along a thirdlongitudinal axis, generally parallel to the first and secondlongitudinal axes, a second elongate light source extending along afourth longitudinal axis, generally parallel to the first, second andthird longitudinal axes, the second elongate light source being arrangedwith respect to the axial aperture of the cylindrical reflector suchthat light from the second elongate light source is directed through theaxial aperture onto the plane generally along the third longitudinalaxis, and an optical element disposed along a light path between thesecond elongate light source and the plane and being operative to passlight from the second elongate light source to the plane and to reflectlight from the plane to an optical sensor.

[0032] Further in accordance with a preferred embodiment of the presentinvention the first elongate light source comprises an elongate lightgenerating element and an elongate reflector extending along a fifthlongitudinal axis, generally parallel to the first, second, third andfourth longitudinal axes and being operative to reflect light from theelongate light generating element onto the cylindrical reflector.

[0033] There is also provided in accordance with yet another preferredembodiment in accordance with a preferred embodiment of the presentinvention an illumination system including at least two elongate lightsources extending along at least two light source longitudinal lightsource axes, each of the at least two elongate light sources beingarranged to illuminate a plane generally along a longitudinalillumination axis, generally parallel to the at least two light sourcelongitudinal axes, and an optical element disposed along a light pathbetween at least one of the at least two elongate light sources andhaving astigmatism, the optical element being operative to cause lightfrom the at least one of the at least two elongate light sources to besmeared along the longitudinal illumination axis.

[0034] There is also provided in accordance with another preferredembodiment of the present invention an illumination system including alight source arranged to illuminate an object, a plurality of lensesimaging light from the light source reflected by the object onto aplurality of light receivers, the physical sizes of the plurality oflenses and the spacing of the plurality of light receivers being suchthat the plurality of lenses cannot all lie in a single plane, foldingoptics arranged along light paths between the plurality of lenses andthe plurality of light receivers and being arranged such thatnotwithstanding different physical separations between ones of theplurality of lenses and corresponding ones of the plurality of lightreceives the light paths between the ones of the plurality of lenses andthe corresponding ones of the plurality of light receivers aresubstantially identical.

[0035] Further in accordance with a preferred embodiment of the presentinvention the plurality of lenses have substantially identical focallengths.

[0036] Additionally or alternatively the plurality of lenses image atleast partially overlapping regions on the object.

[0037] There is further provided in accordance with a preferredembodiment of the present invention an inspection device including firstand second light sources arranged to illuminate a substrate havingreflective features thereon, the first and second light sourcesproviding light outputs having wavelength spectra which are at mutuallydistinguishable, and at least one light receiver receiving light fromthe first light source reflected from the substrate and receiving lightfrom the second light source transmitted via the substrate.

[0038] There is further provided in accordance with a preferredembodiment of the present invention imaging apparatus including anilluminator illuminating a region on the surface of an article. Aplurality of sensors is associated with a plurality of quasi-telecentriclenses, and the sensors and lenses are arranged to simultaneously imagemutually overlapping portions of the illuminated region. Preferably,principal rays imaging each part of the portion deviate less than ±10°from the normal, and more preferably they deviate less than or equal to3.5° from the normal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present invention will be understood and appreciated morefully from the following detailed description, taken in conjunction withthe drawings, in which:

[0040]FIG. 1 is a simplified schematic illustration of an illuminationand imaging system constructed and operative in accordance with apreferred embodiment of the present invention;

[0041]FIG. 2 is a simplified side view of the illumination and imagingsystem shown in FIG. 1;

[0042]FIG. 3 is a simplified schematic illustration of aquasi-telecentric imaging system for simultaneously imaging mutuallyoverlapping regions using multiple lenses in accordance with a preferredembodiment of the present invention;

[0043]FIG. 4 is a simplified ray diagram of the quasi telecentricimaging system shown in FIG. 3;

[0044]FIG. 5 is a simplified side view of a preferred embodiment of aquasi-telecentric imaging system of the type shown in FIG. 3 and FIG. 1;and

[0045]FIG. 6 is a simplified pictorial illustration of an imaging systemfor imaging a patterned object having voids, using illumination indistinguishable spectral ranges in accordance with a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Reference is now made to FIG. 1 which is a simplified schematicillustration of an illumination and imaging system 10 constructed andoperative in accordance with a preferred embodiment of the presentinvention, and to FIG. 2 which is a simplified side view of the systemof FIG. 1. Illumination and imaging system 10 typically forms part of anautomated optical inspection system which comprises a first imageprocessing subsystem running in hardware, preferably as described incopending Israel Patent Application 131092, and a second imageprocessing subsystem running in software as described in copendingIsrael Patent Application filed concurrently herewith.

[0047] The illumination and imaging system of FIG. 1 can be used for theinspection of patterns on flat surfaces of articles such as electricalcircuits. Such electrical circuits, include for example, printed circuitboards, ball grid array substrates, multi-chip modules andsemiconductors. Other examples of articles that can be inspected usingthe system of FIG. 1 include reticles, lead frames and finely engravedand etched metal substrates, as may be found, for example, on medicalimplants. Reference herein to a patterned article is deemed to includeany suitable patterned article.

[0048] Illumination and imaging system 10 is preferably operative toilluminate a portion of a surface 12 of a patterned article which isbeing inspected. A first reflector 14, preferably formed from a sectionof a cylinder having a substantially elliptical cross-section, isoriented with respect to surface 12 so as to subtend an angle α,preferably, approximately ±24° with respect to a location 13 thereon,location 13 being at or near a first focus of first reflector 14.Reflector 14 includes a centrally disposed light transmissive region,preferably an aperture 16, overlying location 13.

[0049] A first illuminator 18 is situated at or near a second focus offirst reflector 14, and is configured and arranged to directillumination onto first reflector 14 and therefrom onto surface 12 atlocation 13 to define a linear illuminated portion 20 of surface 12.First illuminator 18 preferably includes a bundle of 80 micron opticalfibers available from Schott Corporation of Germany which are fanned outto form a fiber optic emitter preferably having dimensions of 1 mm by100 mm. First illuminator 18 is preferably fed by a halogen projectionlamp.

[0050] Preferably a planar folding mirror 22, operative to foldillumination emitted by first illuminator 18, is disposed betweenreflector 16 and first illuminator 18 as shown in FIG. 1.

[0051] A second reflector 24, preferably formed from a section of acylinder having a substantially elliptical cross-section, is disposedand arranged to overlie aperture 16 so that a first focus of reflector24 lies at or near linear illuminated portion 20.

[0052] A second illuminator 26, the intensity of which preferably isadjustable independently of the intensity of first illuminator 18, isprovided at a second focus of reflector 24, and is arranged to directillumination onto second reflector 24 and from second reflector 24through aperture 16 onto linear illuminated region 20. Second reflector24 and second illuminator 26 preferably are configured and arranged sothat illumination from second illuminator 26 slightly overfills aperture16. As a result, a solid angle of illumination, part of which isprovided by illuminator 18 and part of which is provided by illuminator26, fills the entire extent of angle a.

[0053] At least one, and preferably three, imaging sensors 30, arearranged to image linear illuminated region 20 via aperture 16.Quasi-telecentric imaging optics 32, described hereinbelow in greaterdetail with reference to FIG. 4-FIG. 6, preferably are provided. Sensors30 may be CCD or TDI sensors, and are preferably KLI 2103 Red, Green andBlue sensor units, available from Eastman Kodak. These sensors arepreferably operative to acquire polychromatic images and outputreflective intensities for Red, Green and Blue spectra in the linearilluminated region 20. Preferably, patterned surface 12 is positioned ona stage (not shown) which is transported below reflector 14 in a scandirection 34, while sensors 30 image the linear illuminated region 20.

[0054] An optical element 40, preferably a highly optically flat lighttransmissive plane parallel plate, preferably approximately equal inlength to second reflector 24, is disposed between aperture 16 andsecond reflector 24, defines an optical path, part of which extendsalong an axis 42 normal to surface 12, along which sensors 30 imagelinear illuminated region 20. Preferably, optical element 40 is arrangedat a 45° angle relative to the plane of surface 12, and is provided witha partially silvered surface 44 on the underside thereof. Opticalelement 40 preferably transmits light received from second illuminator26 via reflector 24 onto linear illuminated region 20 and directs lightthat is reflected from linear illuminated region 20 to sensors 30.

[0055] It is readily appreciated from the preceding discussion that thepresent invention provides a solid angle of intense illumination to alinear region of a workpiece using only two separately adjustablesources of illumination. First illuminator 18 and second illuminator 26each provide illumination that is oriented along axis 42. The solidangle of the illumination provided by illuminator 26 is narrower thanthat of the illumination provided by illuminator 18.

[0056] It is a particular feature of the present invention that opticalelement 40 serves two disparate functions. Principally, it reflectslight received from linear illuminated region 20 onto sensors 30substantially without astigmatism. Secondarily it introduces astigmatisminto light transmitted therethrough from reflector 24 onto linearilluminated region 20. This astigmatism is perpendicular to thelongitudinal axis of linear illuminated region 20 and produces desiredsmearing of the illumination from reflector 24. The extent of thedesired smearing is controlled by suitable orientation and the thicknessof the optical element 40 relative to reflector 24 and surface 12.

[0057] Reference is now made to FIG. 3, which is a simplified schematicillustration of a quasi-telecentric imaging system for simultaneouslyimaging mutually overlapping regions using multiple lenses in accordancewith a preferred embodiment of the present invention.

[0058] In systems for automated optical inspection of surfaces having anon-even surface topography, as is generally present in printed circuitboards, ball grid array substrates, lead frames, reticles, and otheretched and engraved surfaces, it is generally desirable to employ animaging system which has a high degree of telecentricity. A telecentricimaging system is an optical system having an entrance pupil located atinfinity. In telecentric imaging systems, the principal ray emanatingfrom each point on a surface being imaged is substantially parallel tothe optical axis of the optical system. Consequently, all of the pointson the surface are viewed at the same perspective. In order to achieve ahigh degree of telecentricity, the front element of a lens in theoptical system is typically at least as large as its field of view.

[0059] In conventional imaging systems a high degree of telecentricityis typically obtained either by employing very large and expensivelenses to image an entire region, or by employing a plurality of smallerand less expensive lenses wherein each individual smaller lens images arelatively small region and the plurality of lenses together image theentire region.

[0060] Because of the physical size requirements of telecentric lensesas described hereinabove, in conventional multiple telecentric lensconfigurations, the individual lenses are spaced apart from each otherand image non-mutually overlapping regions. Thus, in conventionaltelecentric imaging systems, in order to image an entire surface, it isnecessary either to scan the surface in multiple passes or to staggermultiple rows of telecentric lenses.

[0061] As seen in FIG. 3, a quasi-telecentric imaging system 310 isprovided for simultaneously imaging mutually overlapping regions usingmultiple lenses which are configured and arranged in accordance with apreferred embodiment of the present invention.

[0062] Quasi-telecentric imaging system 310, includes at least two, andpreferably three or more, RGB sensor units 332, 334 and 336, whichtogether make up sensor 30 (FIG. 1). In the preferred arrangement shown,each of the three sensors 332, 334 and 336 is operative tosimultaneously view a corresponding one of a mutually overlappingviewing region 352, 354, and 356 along surface 12 (FIG. 1), viarespective lenses 362, 364 and 366 associated therewith. Preferablylenses 362, 364 and 366 are Macrosimar 120 mm, F5.6 lenses.

[0063] Sensors 332, 334 and 336 and lenses 362, 364 and 366 arepreferably arranged so that overlapping viewing regions 352, 354 and 356intersect linear illuminated region 20 (FIG. 1). It is readilyappreciated that only those parts of viewing regions 352, 354 and 356which fall along linear illuminated region 20 are imaged by sensors 332,334 and 336.

[0064] In order to explain the structure of FIG. 3, reference is madeadditionally to FIG. 4, which is a simplified optical diagram of quasitelecentric imaging system 310. Rays 400 are shown in the diagram assubtending an angle β which is exaggerated for the sake of simplicityand clarity of illustration. In a quasi telecentric imaging system suchas that of the present invention, the maximum deviation from the opticalaxis 42 (FIG. 1), normal to surface 12 (FIG. 1) of the principal raysreaching sensors 332, 334 and 336 from portions of respective viewingregions 352, 354 and 356 is less than ±5°, and more preferably less thanor equal to ±3.7°.

[0065] Each of telecentric lenses 362, 364 and 366 is preferably held bya respective mounting, designated by respective reference numerals 462,464 and 466, and is located an equal optical distance from surface 12and sensors 432, 434 and 436. Together with their respective mountings,lenses 362, 364 and 366 are typically larger than respective viewingregions 352, 354 and 356.

[0066] In order to provide simultaneous viewing of mutually overlappingviewing regions 352, 354 and 356 and to accommodate lenses 362, 364 and366 a first planar folding mirror 470 is preferably interposed betweentelecentric lens 366 and viewing region 354. A second planar foldingmirror 478, interposed between first planar folding mirror 470 andtelecentric lens, is optionally provided. Planar folding mirror 470 ispreferably sufficiently large to provide an unhindered view of viewingregion 354, and is arranged to be situated intermediate rays 472 and 476entering respective lenses 362 and 364 so as not to interfere with theirview of mutually overlapping viewing regions 352 and 356. It is notedthat first planar folding mirror 470 folds the rays that are shown inFIG. 4 as extending between overlapping portion 354 and sensor unit 334.Thus it is appreciated that sensor unit 334 and telecentric lens 366 andmountings 464 lie in a different plane than sensor units 332 and 336,telecentric lenses 362 and 364, and mountings 462 and 466 respectively.

[0067] Reference is made to FIG. 5 which is a simplified side viewillustration of an illumination and imaging system 510 for illuminatingand imaging a linear illuminated region on a flat patterned article 512.The embodiment of FIG. 5 is an alternative to that of FIGS. 1-4 and isbelieved to be the preferred embodiment. The illumination and imagingsystem 510 preferably employs quasi-telecentric imaging combined with anillumination system which is identical to that described hereinabovewith reference to FIG. 1 and FIG. 2.

[0068] Illumination and imaging system 510 preferably includes a firstreflector 514, preferably formed from a section of a cylinder having asubstantially elliptical cross-section, which is oriented with respectto a surface 512 of an article so as to subtend and angle α, preferably±24°, with respect to a location thereon 513, which is arranged to be ator near a first focus of reflector 514. First reflector 514 preferablyincludes a centrally disposed light transmissive region, preferably anaperture 516, overlying location 513.

[0069] A first illuminator 518 is situated at or near a second focus offirst reflector 514 and is configured and arranged to directillumination onto first reflector 514 and therefrom onto surface 512 atlocation 513 to define a linear illuminated portion 520 of surface 512.First illuminator 518 is preferably configured and arranged similarly toilluminator 18 as described hereinabove with reference to FIG. 1.Preferably a planar folding mirror 522, operative to fold illuminationemitted by first illuminator 518, is disposed between reflector 514 andfirst illuminator 518.

[0070] A second reflector 524, preferably formed of a section of acylinder having a substantially elliptical cross-section, is disposedand arranged to overlie aperture 516 so that a first focus of reflector524 lies at or near linear illuminated portion 520.

[0071] A second illuminator 526, the intensity of which preferably isadjustable independently of the intensity of first illuminator 518, isprovided at a second focus of reflector 524, and is arranged to directillumination onto second reflector 524 and from second reflector 524through aperture 516 onto linear illuminated region 520. Secondreflector 524 and second illuminator 526 preferably are configured andarranged so that illumination from second illuminator 526 slightlyoverfills aperture 516.

[0072] Preferably three imaging sensors 530 (of which only two areseen), are arranged to image linear illuminated region 520 via aperture516. Sensors 530 may be CCD or TDI sensors, and are preferably KLI 2103Red, Green and Blue sensor units, available from Eastman Kodak. Thesesensors are preferably operative to acquire polychromatic images and tooutput reflective intensities for Red, Green and Blue spectra in thelinear illuminated region 20.

[0073] An optical element 540, preferably a highly optically flat lighttransmissive plane parallel plate, is disposed between aperture 516 andsecond reflector 524, defines an optical path, part of which extendsalong an axis 542, normal to surface 512, along which sensors 530 imagelinear illuminated region 520. Preferably, optical element 540 isarranged at a 45° angle relative to the plane of surface 512, and isprovided with a partially silvered surface 544 on the underside thereof.Optical element 540 preferably transmits light from second illuminator526 via reflector 524 to linear illuminated region 520 and directs lightthat is reflected from linear illuminated region 520 to sensors 530.

[0074] Each sensor 530 is preferably associated with a quasi-telecentricimaging lens 558. Each sensor 530 is operative to simultaneously image amutually overlapping portion of linear illuminated region 520.

[0075] A first planar folding mirror 580 is disposed along a firstoptical path 582, shown in broken lines, extending from linearilluminated region 520 to a first sensor 530 via a first lens 558. Asecond planar folding mirror 584 is disposed in a second optical path586, shown in solid lines, extending between linear illuminated region520 and a second sensor 530 via a second lens 558. A third planarfolding mirror (not seen in FIG. 5) is disposed in a third optical path(not seen in FIG. 5) which is hidden by optical path 586, extendingbetween linear illuminated region 520 and a third sensor 530 via a thirdlens 558, both of which are not seen in FIG. 5.

[0076] It is noted that first planar folding mirror 580 is preferablysufficiently large to provide an unhindered view of a second portion oflinear illuminated region 520 and is arranged to be situatedintermediate optical paths entering respective first and third lenses558 so as not to interfere with their view of mutually overlappingportions of linear illuminated region 520.

[0077] It is additionally noted that despite the different locations ofthe first, second and third mirrors, the respective optical pathsthrough the first, second and third lenses 558 are each situated at anequal optical distance from the respective sensor 530, and each lens 558is situated at an equal optical distance from linear illuminated region520.

[0078] Reference is now made to FIG. 6, which is a simplified pictorialillustration of an imaging system 610 for imaging a patterned objecthaving apertures, using illumination in mutually distinguishablespectral ranges in accordance with a preferred embodiment of the presentinvention. The imaging system 610 is described herein with reference tothe inspection of lead frames, for which its is particularly suited. Itis readily appreciated that the imaging system 610 is not limited to usein inspecting lead frames and is generally useful for opticallyinspecting any suitable patterned surface that is characterized byapertures, voids and through holes.

[0079] Lead frames typically comprise a plurality of leads 612, whichhave a width and height dimension and which are typically formed via asuitable pressing, engraving or etching process, and which are typicallycoated with a metal coating. A typical lead frame is formed of copper,and coated at various locations thereon with silver and gold. Leads 612are generally optically inspected to determine that each of the leads isseparated one from another by a separation 614, and that no pair ofadjacent leads is connected by a bridge 616.

[0080] It is readily appreciated due to the depth dimension of leads612, and due to the existence of non-vertical edges between adjacentleads, which are generally the result of pressing, engraving and etchingprocesses used to manufacture lead frames, the intensity of reflectedlight in regions 618 between leads 612 is typically considerably lessthan the intensity of light reflected from the horizontal regions of theleads. Accordingly, the intensity of light reflected from a bridge 616may be very close to zero as is characteristic of regions having aseparation 614 between adjacent leads.

[0081] In a preferred embodiment of the present invention, anilluminator is preferably provided to illuminate a first side 640 ofleads 612 in or more spectral ranges which facilitate the detection ofcombinations of materials commonly present on lead frames. For example,a first illuminator 642 is preferably configured to providesubstantially red and green illumination, which together generallyfacilitate the identification of regions of copper, gold and silver.

[0082] A second illuminator, preferably emitting in a spectral rangewhich is distinguishable from the spectral range of first illuminator642, is provided to illuminate a second side 644 of leads 612. Forexample, a second illuminator 646 is configured to provide substantiallyblue illumination, or illumination in a non-visible spectrum, to secondside 644.

[0083] A sensor 650 is preferably provided to view a portion of firstside 640, and is operative to separately sense the intensity of light ineach of the spectral ranges emitted by first illuminator 642 and bysecond illuminator 646. Thus, sensor 650 is operative to sense theintensity of light provided by first illuminator 642 and reflected bythe first side 640, and to separately sense light provided by secondilluminator 646 and transmitted via apertures located in and betweenleads 612, for example light transmitted through separations 614.

[0084] It is readily appreciated that because the spectrum of reflectionof light provided by first illuminator 642 is distinguishable from thespectrum of light provided by second illuminator 646, it is possible tosimultaneously provide an output indicating the existence of voids andapertures in and between leads 612, and to inspect leads for defectswhich are detectable by analyzing reflected light, for example, defectsin coatings on leads 612.

[0085] It is appreciated that various features of the invention whichare, for clarity, described in the context of separate embodiments mayalso be provided in combination in a single embodiment. Conversely,various features of the invention that are, for brevity, described inthe context of a single embodiment may also be provided separately or inany suitable combination. The scope of the present invention is notlimited to what has been described hereinabove but rather also includesmodifications and variations thereof as would occur to persons skilledin the art upon reading the foregoing description and which are not inthe prior art.

1. An illumination system comprising at least one reflector subtendingan angle with respect to a location on a surface of an article; andfirst and second light sources, said first and second light sources eachproviding a light output, the light outputs from both of said first andsecond light sources being directed to impinge on said location on saidsurface of an article within said angle, at least one of said lightoutputs being reflected by said reflector.
 2. An illumination systemaccording to claim 1 and wherein said reflector is formed with a lighttransmissive region to permit a light output from a first one of a saidfirst and second light sources to pass therethrough.
 3. An illuminationsystem according to claim 2 and wherein said light transmissive regionis an aperture formed in the said reflector.
 4. An illumination systemaccording to claim 2 and also comprising an optical element disposedintermediate said first one of said first and second light sources andsaid aperture for directing the light output of said first one of saidfirst and second light sources through said aperture.
 5. An illuminationsystem according to claim 4 and wherein said optical element includes atleast one reflecting surface.
 6. An illumination system according toclaim 4 and also comprising an optical assembly disposed along anoptical path extending between said optical element and said location onthe surface of an article for directly light reflected from saidlocation on the surface of an article to a light sensor.
 7. Anillumination system according to claim 6 and wherein said opticalassembly is generally transmissive to said the light output of saidfirst one of said first and second light sources directed through saidaperture onto said location on the surface of an article and generallyreflective of light reflected from said location on the surface of anarticle.
 8. An illumination system according to claim 4 and alsocomprising a reflective surface disposed intermediate a second one ofsaid first and second light sources and said reflector for directing thelight output of said second one of said first and second light sourcesonto said reflector.
 9. An illumination system according to claim 1 andwherein said at least one reflector is an elliptical reflector and saidsecond one of said first and second light sources and said location onthe surface of an article are each located at a locus of said ellipticalreflector.
 10. An illumination system according to claim 9 and whereinsaid at least one reflector comprises two spaced apart reflectors whichtogether define a section of an ellipse.
 11. An illumination systemaccording to claim 1 and wherein said first and second light sources areindependently controllable light sources.
 12. An illumination systemcomprising: a cylindrical reflector extending along a first longitudinalaxis and having an axial aperture formed therein which extends alongsaid longitudinal axis; a first elongate light source extending along asecond longitudinal axis, generally parallel to said first longitudinalaxis and being arranged to illuminate said cylindrical reflector, suchthat light from said first elongate light source is directed onto aplane generally along a third longitudinal axis, generally parallel tosaid first and second longitudinal axes; a second elongate light sourceextending along a fourth longitudinal axis, generally parallel to saidfirst, second and third longitudinal axes, said second elongate lightsource being arranged with respect to said axial aperture of saidcylindrical reflector such that light from said second elongate lightsource is directed through said axial aperture onto said plane generallyalong said third longitudinal axis; and an optical element disposedalong a light path between said second elongate light source and saidplane and being operative to pass light from said second elongate lightsource to said plane and to reflect light from said plane to an opticalsensor.
 13. An illumination system according to claim 12 and whereinsaid first elongate light source comprises an elongate light generatingelement and an elongate reflector extending along a fifth longitudinalaxis, generally parallel to said first, second, third and fourthlongitudinal axes and being operative to reflect light from saidelongate light generating element onto said cylindrical reflector. 14.An illumination system comprising: at least two elongate light sourcesextending along at least two light source longitudinal light sourceaxes, each of said at least two elongate light sources being arranged toilluminate a plane generally along a longitudinal illumination axis,generally parallel to said at least two light source longitudinal axes;and an optical element disposed along a light path between at least oneof said at least two elongate light sources and having astigmatism, saidoptical element being operative to cause light from said at least one ofsaid at least two elongate light sources to be smeared along saidlongitudinal illumination axis.
 15. An inspection device comprising:first and second light sources arranged to illuminate a substrate havingreflective features thereon, said first and second light sourcesproviding light outputs having wavelength spectra which are at leastpartially different; and at least one light receiver receiving lightfrom said first light source reflected from said substrate and receivinglight from said second light source transmitted via said substrate. 16.Imaging apparatus comprising: at least one illuminator illuminating aregion on the surface of an article; and a plurality of sensorsassociated with a plurality of lenses, said sensors and lenses arrangedto simultaneously provide quasi-telecentric images of mutuallyoverlapping portions of the illuminated region.
 17. Imaging apparatusaccording to claim 16 and wherein the principal rays imaging each partof the portion deviate less than ±5° from the normal.
 18. Imagingapparatus according to claim 16 and wherein the principal rays imagingeach part of the portion deviate less than or equal to 3.7° from thenormal.